The present invention is directed to an optical star coupler for multi-mode optical fibers in which two bundles of optical fiber elements are connected to the input and output surfaces of a mixer element formed by a planar waveguide having a thickness which is approximately equal to the diameter of the fiber elements. The mixer element and the two bundles of fiber elements are disposed in the same plane with the fiber elements lying in a single layer with as maximum a packing density as possible and the fiber elements having a diameter which is approximately equal to the core diameter of the system fibers which are to be connected to the coupler.
For optical broad band communication, optical components are required which distribute the power of any one of the n incoming channels to n outgoing channels. For n&gt;2, these components are called star couplers. These couplers are intended to have as low as possible insertion losses and are to distribute the power as uniformly as possible over the n outgoing channels independent of the type of excitation. For a broad band - x - length - product greater than several 10 MHz xkm, the gradient fibers are the only transmission medium considered for the multi-mode fibers that act as a transmission medium.
Different structures for star couplers are known. For example, the star coupler can have a mixing zone which can consist either of fused together fibers, of tapered and fused together fibers, of a planar waveguide cladded fiber band or of a planar non-cladded glass plate such as disclosed in the U.S. patent application Ser. No. 136,423 filed Apr. 2, 1980, which issued as U.S. Pat. No. 4,362,357 and was based on German Pat. No. P 29 15 114.2. For the manufacture of the mixing zone consisting of fused together fibers, the cladding of the fibers in the mixing region must be removed prior to the step of fusing together. Due to the coupling mechanism, mixer zones consisting of fused together fibers or tapered and fused together fibers are predominantly suitable for only fibers having a stepped profile. These mixers basically couple out in a mode dependent fashion whereby in the case of gradient fibers, this mode dependency has a particular disadvantageous effect. This mode dependency has a consequence that the light in the incoming fibers is not uniformly distributed over n outgoing fibers. By increasing the length of the coupling region, the uniformity of the power distribution will be improved. However, with the longer coupling region, the losses are also simultaneously increased. Moreover it is difficult to manufacture the cited mixer as a sturdy component. An embedding of the fibers in a cement in order to achieve a good, mechanical strength creates problems because in the mixer region, the light reaches the cement and the losses are therefore further increased. Thus, when the coupler, which is disclosed in the above mentioned copending application, is utilized with a gradient system without special measures, it will have high insertion losses or insufficient division of the power equally between the outgoing fibers.